A duplexer of a base transceiver module is formed of RF (radio frequency) cavity filters, is generally located on a mechanical part on a rear side of a transceiver board, and is used for transmitting a single-path high-power signal. The RF cavity filters in the duplexer include a TX filter (transmit channel filter) and an RX filter (receive channel filter). With the development of multi-carrier and multi-standard mobile communications, a demand for platformization gradually grows. For example, a frequency band of DCS (distributed control system), PCS (personal communications service), TD-SCDMA (Time Division Synchronous Code Division Multiple Access), and UMTS (Universal Mobile Telecommunications System) is 1710 to 2170 MHz, and therefore a large number of duplexers of different frequency sub-bands are required. If a filter is bandwidth-tunable, signals of different frequency can be tuned by one duplexer, which is of great significance to improving platformization of a duplexer and saving management and manufacturing costs
FIG. 1 shows a single-cavity model of a cavity filter in the prior art, where a resonator 102 is fastened onto a step 101, the resonator 102 is equivalent to an inductor L, an upper surface of the resonator 102 and a cover connected to the top of a tuning bolt 103 are equivalent to a capacitor C, and an equation for calculating resonance frequency Fr of a single cavity is
  Fr  =            1              2        ⁢        π        ⁢                  LC                      .  To make the filter bandwidth-tunable, it needs to be ensured that the resonance frequency of each single cavity is tunable, that is, enabling the equivalent inductor L or the equivalent capacitor C to be tunable or both of them to be tunable. To achieve the objective, the prior art provides various solutions, and examples thereof are given as follows:
As shown in FIG. 2, a US patent US20090058563 discloses a tunable filter. In a dielectric TE (transverse electric wave) mode filter, dielectric tuning parts 203 and sliding members 204 are added between a cavity 201 and a cover 202. By moving bolts 205 that fasten the dielectric tuning parts 203 inside guiding grooves, cavity perturbation between the dielectric tuning parts 203 and dielectric resonators 206 is achieved, thereby changing the resonance frequency. However, because a tuning range of the dielectric filter is relatively small and a tuning range of the dielectric parts is much more limited, the tuning range of such a filter still cannot meet requirements.
As shown in FIG. 3, a Chinese patent with an application number of 201110251164.8 discloses a tunable filter, where an inductance coil 302 is wound over a tuning bolt 301, and currents of different directions and magnitudes are supplied to the inductance coil 302 so as to change inductance, thereby achieving cavity perturbation and further achieving tuning. Although this technical solution can achieve a multi-band filtering function without manual tuning, a position where the tuning bolt and the cover are connected is grounded, and applying a voltage nearly has little influence on the inductance of the inductance coil. In addition, the cavity filter is a typical distributed constant circuit filter and the inductance coil is a low-frequency lumped parameter component; therefore, it is difficult to tune the cavity resonance frequency by changing a inductance value and a current direction of a lumped parameter inductor at high frequency. As a result, the tuning range of the filter is still small, and cannot meet a requirement of tunable wideband, which results in that platformization cannot be achieved.